Ethernet standards now support a large range of different line speeds, from 10 to 1000 MHz, with the option of operating in either full duplex or half duplex mode. IEEE Standards 802.3 and 802.3ab define an auto-negotiation process by which a pair of Ethernet nodes at opposite ends of a link may advertise and negotiate the link speed and duplex mode (full or half) to be used on the link. A useful overview of the auto-negotiation process is provided by Hernandez in “Gigabit Ethernet Auto-Negotiation,” Dell™ Power Solutions (Issue 1, 2001), pages 117-122, which is incorporated herein by reference.
Unfortunately the auto-negotiation process is not foolproof and sometimes results in duplex mismatch, i.e., situations in which the device at one end of a link is set for full-duplex operation and the other device is set for half-duplex. In this case, when both devices attempt to send frames over the link simultaneously, the half-duplex device will detect a collision on the link. Upon detecting the collision, the half-duplex device discards the incoming frame it has received and subsequently attempts to retransmit its own outgoing frame. The full-duplex device, on the other hand, may detect an error in its incoming frame but does not retransmit its own outgoing frame. This deviant link behavior causes applications using the link to time out and retransmit continually. As a result, the effective connection speed of the link is degraded.
Various tools are known in the art for automated testing of packet network performance. For example, U.S. Pat. No. 5,812,529, whose disclosure is incorporated herein by reference, describes a system and method for acquiring network performance data, built around a “mission server,” which interfaces with clients to receive requests for “missions.” A typical mission includes operations such as transmission and reception of data packets among devices connected to segments of the network. The mission is performed and/or supported by “sentries,” typically software agents running on stand-alone network devices or end-points. The sentries carry out mission operations in response to commands from the mission server, and report back to the mission server on the mission results.
U.S. Pat. Nos. 5,838,919 and 5,881,237, whose disclosures are incorporated herein by reference, describe methods, systems and computer program products for testing of network performance using test scenarios that simulate actual communications traffic between network endpoints. Specific test protocols are assigned to endpoint nodes on the network. Typically, the nodes are paired, and one of the nodes in the pair communicates the protocol to the other, associated node. A console node sets up the test protocols, initiates their execution and receives data on the test performance from the endpoint nodes.
U.S. Pat. No. 6,269,330, whose disclosure is incorporated herein by reference, describes a method and apparatus for testing a network having a plurality of nodes. The method includes sending commands to one or more traffic agents connected to the network and to at least one network management agent coupled to a respective node of the network, transmitting data from at least one of the traffic agents over the network responsive to the commands, determining network information at the at least one network management agent responsive to the commands and to transmission of the data through the respective node, and receiving and evaluating the network information to assess a state of the network. Aspects of the methods described in U.S. Pat. No. 6,269,330 are embodied in an Active Testing Framework (ATF) known as NetAlly™, available from Viola Networks (Yokneam, Israel).
PCT Patent Publication WO 01/82022 A2, whose disclosure is incorporated herein by reference, describes a method for testing of a communication network using a plurality of traffic agents. The method includes transmitting a sequence of data packets via the network from a first traffic agent to a second traffic agent, and recording arrival characteristics of the packets responsively to receiving the packets at the second traffic agent. The arrival characteristics of different packets in the sequence are compared so as to determine a measure of variability in transmission of the packets via the network.
PCT Patent Publication WO 01/95053 A2, whose disclosure is incorporated herein by reference, describes a method for testing of a communication network using one or more network agents coupled to the network at respective locations. Data packets meeting a specified packet filtering criterion are transmitted through the network from one of the end-point to another. Packets meeting the criterion are intercepted using the network agents at one or more of the respective locations in the network traversed by the at least one of the data packets. Information regarding the intercepted packet is recorded and processed in order to analyze a route of the packets through the network.